One or more layers of refractory material are often applied to cutting tool surfaces by physical vapor deposition (PVD) techniques to increase properties including wear resistance, performance and lifetime of the cutting tool. Titanium nitride (TiN) coatings, for example, are commonly applied by PVD to cemented carbide cutting tool substrates. However, TiN begins oxidation at about 500° C. forming rutile TiO2, thereby promoting rapid coating deterioration. Incorporation of aluminum into the cubic lattice can slow degradative oxidation of a TiN coating by forming a protective aluminum-rich oxide film at the coating surface.
While providing enhancement to high temperature stability, aluminum can also induce structural changes in a TiN coating having a negative impact on coating performance. Increasing amounts of aluminum incorporated into a TiN coating can induce growth of hexagonal close packed (hcp) aluminum nitride (MN) phase, altering the crystalline structure of the coating from single phase cubic to a mixture of cubic and hexagonal phases. In some instances, aluminum content in excess of 70 atomic percent can further alter the crystalline structure of the AlTiN layer to single phase hcp. Significant amounts of hexagonal phase can lead to a considerable reduction in hardness of AlTiN, resulting in premature coating failure or other undesirable performance characteristics. Difficulties in controlling hexagonal phase formation can obstruct full realization of the advantages offered by aluminum additions to TiN coatings.